gn: Fix build issues blocking gfx from being built

third_party/libpng/BUILD.gn

Index: third_party/libpng/BUILD.gn
diff --git a/third_party/libpng/BUILD.gn b/third_party/libpng/BUILD.gn
index f920c0ba3cb8e88d79c7055104941836332eac75..0d2eeb4c962a867b92ec393d0f4262d2760a54ef 100644
--- a/third_party/libpng/BUILD.gn
+++ b/third_party/libpng/BUILD.gn
@@ -29,7 +29,7 @@ config("libpng_config") {
   }
 }
 
-static_library("libpng") {
+source_set("libpng_sources") {
   sources =  [
     "png.c",
     "png.h",
@@ -58,13 +58,23 @@ static_library("libpng") {
 
   if (is_win) {
     cflags = [ "/wd4267" ]  # TODO(jschuh): http://crbug.com/167187
-  }
 
-  forward_dependent_configs_from = [ "//third_party/zlib" ]
+    if (component_mode == "shared_library") {
+      defines = [ "PNG_BUILD_DLL" ]
+    }
+  }
 
   direct_dependent_configs = [ ":libpng_config" ]
 
-  deps =  [
-    "//third_party/zlib",
-  ]
+  public_deps = [ "//third_party/zlib" ]
+}
+
+if (is_win) {
+  component("libpng") {
+    public_deps = [ ":libpng_sources" ]
+  }
+} else {
+  static_library("libpng") {
+    public_deps = [ ":libpng_sources" ]
+  }
 }